RESTITUTION PHASE 139 



must be followed by a stage in which energy is again stored for 

 work. During this phase four things are evident, viz. : 



I. Oxygen is necessary. 



II. Carbon-dioxide is evolved. 



III. Lactic acid disappears. 



IV. Heat is evolved. 



Examination of table XXII. given above will make it clear to 

 the student that I. and II. are different parts of the same reaction. 

 Oxygen is used to oxidise some carbon compound which is excreted 

 as CO 2 ( +H 2 O). If oxygen is not present we learn from the work 

 of Fletcher and Hopkins that the lactic acid set free during the 

 contraction phase is not removed. The amount of heat evolved 

 has been very carefully estimated by Hill and others. They 

 find that it is almost exactly equivalent to the tension developed 

 in contraction. If the tension be estimated in heat units and found 

 to be 5 xlO~ 6 cals. per gram- weight of tension per centimetre of 

 muscle length, the heat evolved during the restitution phase 

 would also be 5 xlO~ 6 cals. per gram-weight. The work done in 

 putting the muscle back into its precontraction state is equal to 

 the tension developed during contraction. 



Two views are held regarding the mechanism of . restitution. 

 The commoner idea is that lactic acid is in muscle as elsewhere 

 an intermediate compound in the oxidation of glucose. Glucose, 

 we know, is stored in muscle and this store is decreased during 

 contraction. The amount of heat liberated from glucose in the 

 formation of lactic acid is very small, while the amount liberated 

 when lactic acid is itself oxidised to CO 2 +H 2 O is considerable. 

 1 gram of lactic acid in oxidation sets free 3700 cals. These 

 facts seem to fit in well to the following scheme. During 

 contraction lactic acid is formed from glucose. For this, very 

 little oxygen is required and very little CO 2 is evolved. Practi- 

 cally no heat appears. During restitution the lactic acid is 

 removed by oxidation and heat is evolved. 



Neat though this theory is, it has been rendered untenable by 

 the brilliant researches of A. V. Hill and V. Weizsacker with their 

 ingenious myothermic apparatus, and by Fletcher and Hopkins 

 who devised a delicate means of estimating lactic acid. When 

 1 gram of lactic acid disappears from muscle, only 450 cals. 

 instead of 3700 are produced. Obviously, only a small fraction 

 of the lactic acid is oxidised if any. Fletcher and Hopkins could 

 not detect any oxidation of lactic acid. 



